Answer: option C. Strong nuclear
Step-by-step explanation:
1) There are four fundamental interactions or forces: electromagnetic force, gravity, strong nuclear, and weak nuclear.
2) Elctromagnetic force is the force between an electric field and a charge. When two charges are equal, either positive or negative, the electromagnetic force is repulsive. This is the force that makes the atom nuclei unstable, because the several protons (from 1 to 118 depending on the element) are together in a tiny space. Hence, it is necessary another stronger force that overcomes the repulsion and keeps the atoms from desintegrating.
3) Gravity is the force between two masses. It is only important when the masses are big (like the masses of the Earth and the Mooon). It is not relevant in the protons because the mass of a proton is very small.
4) Strong nuclear force: is the force that gives stability to the atomic nucleii. The strong nuclear force has a very small range, and it is the very strong at the nucleii sizes, where it is more than 100 times stronger than the electromagnetic repulsive force between the protons.